The present invention generally relates to a system and method for supplying a controlled power to a load and, more particularly, for supplying a constant RMS voltage to a load.
In electronic displays, a backlight is used to illuminate the display for viewing purposes. Many high performance transmissive liquid crystal display (LCD) systems, such as those used in the aircraft and avionics industry, utilize a light source positioned behind the display to enable viewing. The LCD is often xe2x80x9cbacklitxe2x80x9d using a small fluorescent discharge lamp.
Fluorescent lamps typically exhibit the highest level of efficiency (i.e., optimal luminance) when they are operated at a particular ambient temperature, which can vary depending upon the lamp, display setting, and ambient conditions. For example, Honeywell International Inc. has found that many of the fluorescent lamps used in aircraft display systems exhibit optimal behavior when operated around 55xc2x0 C. One common technique for attaining and maintaining a desired lamp temperature includes the use of a heating element having an active control system.
FIG. 1 illustrates a pulse-width modulated power supply system 100 delivering power to a heating element 102 (i.e., the load) of a conventional lamp heating system. Power system 100 includes an active control system having a temperature sensor 104, a comparator 106, and a switching regulator 108. Temperature sensor 104 monitors the temperature of the lamp (not shown); comparator 106 compares the temperature reading to a preset desired temperature (i.e., temperature set point); and, in response to the comparison, regulator 108 controls the amount of DC power supplied to the heating element.
Under normal conditions (e.g., non-extreme weather temperatures), the airplane generators supply approximately 28 volts of DC power. However, the voltage can range from about 18 to 32 volts due to, for example, battery operation and tolerance on the generator. In addition, transient voltage spikes, caused in part by various switching functions, can momentarily increase the voltage to around 80 volts. To help protect the system from destructive voltage spikes and to provide regulated voltage to the heating element, a switching regulator (i.e., regulator 108) is often used.
With some certainty, the pulse-width modulated power system of FIG. 1 produces a constant voltage across heating element 102. Switching regulator 108 typically includes a large number of costly electrical components which require high power input and consume valuable printed wiring board (PWB) area. For example, switching regulator 108 generally requires magnetics, such as transformers and inductors, which are heavy, bulky and often generate unwanted electromagnetic interference (EMI). Moreover, increasing the number of components in a system tends to increase the time required to test the system, repair costs, and the probability of system failure.
FIG. 2 illustrates a power supply system 200 which attempts to solve some of the problems of the pulse-width modulated system, namely by reducing the number of components, weight, and cost. xe2x80x9cOff-linexe2x80x9d power supply system 200 eliminates the switching regulator and provides unregulated power to the load (a heating element 202). Used herein, xe2x80x9coff-linexe2x80x9d refers to right-off-the-line or direct without conversion of power. A temperature sensor 204 and a comparator 206 operate in much the same manner as previously discussed for system 100. The unregulated power (approximately 28 volts) generated by the aircraft is directly connected to heating element 202 with a switching element 208 coupled to the return of the 28 volt power.
In operation, power supply system 200 provides power to heating element 202 as determined by the temperature of the lamp (not shown). In other words, temperature sensor 204 provides temperature readings to comparator 206, which in turn compares the reading to a preset desired temperature (e.g., 55xc2x0 C.) and drives switching element 208 accordingly. The system supplies power to the heating element until the desired temperature is reached and then shuts itself off. It should be noted that hysteresis is inherent in this system, as well as system 100, due in part to thermal lags.
At start-up, the heating element used in aircraft display systems may consume a considerable amount of power and/or an undesirable length of time to reach the operating temperature. Often, especially in colder temperatures, several banks of batteries are used to start the airplane""s systems. The voltage available from the batteries is generally lower than the airplane generators used under normal weather conditions. Thus, preferably the heating element in the power supply system is be able to heat the lamp in the least amount of time and use the least possible amount of DC power.
Current airline regulations require the aircraft, including its systems, to be ready to fly from a resting or off state in fifteen minutes, regardless of the climate. If, for example, a power system (such as system 200) is designed around 28 volts to provide enough heat to warm the airplane""s display in 15 minutes, then when the output voltage drops to 18 volts, the power system is likely not to produce enough power to heat the display in the given time. On the other hand, if the system is designed around 18 volts, then when the output jumps to 32 volts, too much power (heat) is drawn from the aircraft. Most commercial aircraft have multiple display systems each drawing power from the aircraft. Thus, under certain conditions, multiple off-line systems will quickly consume an extreme amount of power and drain the aircraft""s generated power. This situation is intolerable, especially when the aircraft is operating under battery conditions.
The present invention provides a solution to the prior art problems outlined above. According to various aspects of the present invention, a controlled power system supplies a constant RMS voltage to a load and includes a pulse width modulator, a first control loop configured to monitor a characteristic variable, and a second control loop configured to generate a control signal in accordance with a comparison of a duty cycle representation of a drive signal to the load and a duty cycle representation of a received input voltage.